The original 1951 Chevy truck radiator, made by Harrison, did not have a "built-in" pressure relief valve. There is a zero (see below, thanks Tim) pound radiator cap that twists on/off onto the neck on the top radiator tank. Soldered into the neck is a tube that runs down the side of the radiator that allows expanded coolant to escape (overflow) if the radiator is filled too full of coolant (coolant expands with heat). I believe it wasn't until 1954 that a 4 pound pressure cap was used. The old radiators can't stand a lot of pressure so I wouldn't go higher than 4 pounds.
If you do not have an original Harrison radiator, see if you can find identifying numbers or manufacturer. A picture would be nice.
Last edited by buoymaker; Mon Sep 13 2021 06:11 PM.